LOW VERTICAL WIND SHEAR (VWS) AND PRONOUNCED RADIAL OUTFLOW. OUTFLOW
IS BEING ENHANCED ON THE POLEWARD SIDE OF THE CYCLONE INTO THE MID-
LATITUDE FLOW AS WELL. EXCESSIVE VERTICAL WIND SHEAR, ASSOCIATED
WITH A STRONG ZONAL JET RUNNING ACROSS CENTRAL INDIA, IS LOCATED
LESS THAN 4 DEGREES TO THE NORTH OF THE TC. GFS AND NOGAPS FIELDS
INDICATE THAT THE JET WILL SLOWLY LIFT TO AROUND 15N BY TAU 48,
ALLOWING FOR SLOW TO STEADY INTENSIFICATION PRIOR TO LANDFALL. MODEL

GUIDANCE IS LIMITED TO JUST A FEW CONSENSUS MEMBERS AND THERE IS

OVER A 600 NM SPREAD BETWEEN THE SOUTHERNMOST (GFDN) AND NORTHERN-

MOST (WBAR) MEMBER AT TAU 72. BOTH GFS AND NOGAPS, THE REMAINING TWO
CONSENSUS MEMBERS, DROP THE CIRCULATION AFTER THE FIRST 12 HOURS.
THE FORECAST IS SLOWER THAN CONSENSUS, PARTICULARLY FOR THE FIRST 36
HOURS, THEN SPEEDS UP BETWEEN TAU 48 AND 72 AS THE RIDGE STRENGTHENS
TO THE NORTH (AND CONSENSUS SLOWS).

The cyclonic storm ‘WARD’ over southwest and adjoining southeast Bay of Bengal remained practically stationary and lay centred at 1730 hours IST of today, the 11th December 2009 near lat. 8.50 N and long. 84.50 E, about 300 km east-northeast of Batticaloa (Sri Lanka), 550 km southeast of Nagapattinam and 700 km south-southeast of Chennai. The system is likely to move in a northwesterly direction towards Tamil Nadu coast during next 48 hours.
However, satellite derived steering wind in upper troposphere suggests the possibility of northerly/northeasterly movement of the system. Considering all the above, the system is under watch
Under its influence, rainfall at many places with isolated heavy to very heavy falls is likely to commence over coastal Tamilnadu and Puducherry from tomorrow, the 12th December 2009 morning. Subsequently, increase in rainfall activities with scattered heavy to very heavy falls and isolated extremely heavy falls is likely over south coastal Tamilnadu from tomorrow, the 12th December 2009 evening onwards.
Squally winds speed reaching 55-65 kmph gusting to 75 kmph are likely to commence along and off Tamilnadu and Puducherry coasts from tomorrow, the 12th December 2009 morning. Sea condition will be very rough to high along and off these coasts. Fishermen are advised not to venture into the sea along and off these coasts.

Wednesday's low-pressure area over south-west Bay of Bengal has intensified into a depression, according to special bulletin from the Chennai Regional Meteorological Centre.

The system lay centred 750 km south-east of Nagapattinam.It is likely to intensify further and move in a west north-westerly direction towards east coast of Sri Lanka during the next 24 hours.

The US Navy's Joint Typhoon Warning Centre, too, has put out a weather alert notice in the south-west Bay of Bengal as a prevailing weather system showed prospects of intensification.

STORM NUCLEUS

The JTWC on Thursday located the area of associated convection to 470 km east of Colombo, Sri Lanka. Animated visible satellite imagery shows sustained and deepening convection beginning to wrap into and organise around a consolidating low level circulation centre (LLCC, the storm nucleus).

Satellites imagery also picked weak winds at the core of a distinct circulation centre with stronger (27 to 46 km/hr) flow located approximately one degree to the north and south of the LLCC.

The upper level environment is conducive for further development with low vertical wind shear (high shear denoted raised wind speeds with height that kills storms) and enhanced outflow, allowing the system to breathe free and grow.

Given this context, the JTWC has upgraded to ‘fair' the potential for the development of a significant tropical cyclone within the next 24 hours (ranging from a depression and above).

Meanwhile, the Noida-based National Centre for Medium Range Weather Forecasting (NCMRWF) under the Ministry of Earth Sciences said that the in its bulletin that the ‘low' over southwest Bay of Bengal and neighbourhood is likely to move west north-westw and may become well-marked.

Isolated to scattered rainfall activity has been forecast for extreme south peninsular India during next 2-3 days before intensifying thereafter.

There is also increasing forecast consensus that the depression might come under the influence of the opposing westerly flows from a prevailing western disturbance over northwest India. International models are of the view that the Bay of Bengal weather system would still manage to cross into southeast Tamil Nadu and adjoining north Sri Lanka.

After making landfall in this manner, however, the weakened system could be made to stagnate while continuing to rain it down over south Tamil Nadu and adjoining Kerala.

A leading cyclonic circulation tracking model is the view that the westerlies would take the system to the north-northeast over land, i.e. towards north coastal Tamil Nadu and south coastal Andhra Pradesh triggering moderate to heavy rains along the way, including over Chennai.

One other tracking model disagreed with this outlook putting the system along a west-northwest track that would take it into north Kerala/coastal Karnataka before leading it into the adjoining coastal Arabian Sea.

This model tends to believe that the system would have the wherewithal to fend off the westerlies, which would have started weakening over the northwest around that time.

The International Research Institute (IRI) for Climate and Society has said that ‘unusually heavy rainfall' is indicated for southwest Bay of Bengal and coastal Tamil Nadu during the five days ending December 14.

Almost similar forecast has been put out by the US National Centres for Environmental Prediction (NCEP), which sees the rains getting sprayed along the Tamil Nadu coast and further north to south coastal Andhra Pradesh.

WESTERLY INFLUENCE

The European Centre for Medium-Range Weather Forecasts (ECMWF) sees the system stagnating over southern Tamil Nadu after being acted upon by westerlies and anchoring the rain session over entire southern peninsula.

Meanwhile, the NCMRWF said that the western disturbance would affect western Himalayan region during the next 24 hours. Under its influence, hilly regions of northwest India are likely to have scattered precipitation while adjoining plains would witness isolated light showers.

Night temperatures over northwest India are expected to increase during the period given the presence of cloudiness brought about by the western disturbance. But the mercury could plunge later as colder northwesterlies fill the plains.

Model predictions suggest that another Western Disturbance is likely to affect northwest India from December 16 onwards.

The deep depression over southwest and adjoining southeast Bay of Bengal remained practically stationary and lay centred at 0830 hours IST of today, the 11th December 2009 near lat. 7.00 N and long. 84.50 E, about 350 km east-southeast of Batticaloa (Sri Lanka), 650 km southeast of Nagapattinam and 800 km southeast of Chennai. The system is likely to intensify further into a cyclonic storm and move in a west-northwesterly direction and cross Sri Lanka coast between lat. 8.50 N and 9.50 N by evening of tomorrow, the 12th December, 2009. Subsequently, it may emerge into Gulf of Mannar and cross Tamil Nadu coast by noon of 13th December 2009.

Based on latest analysis with Numerical Weather Prediction (NWP) models and other conventional techniques, estimated track and intensity of the system are given in the Table below:

Under its influence, rainfall at many places with isolated heavy to very heavy falls is likely to commence over coastal Tamilnadu and Puducherry from tomorrow, the 12th December 2009 morning. Subsequently, increase in rainfall activities with scattered heavy to very heavy falls and isolated extremely heavy falls is likely over south coastal Tamilnadu from 12th December 2009 evening onwards.

Squally winds speed reaching 55-65 kmph gusting to 75 kmph are likely to commence along and off Tamilnadu and Puducherry coasts from tomorrow, the 12th December 2009 morning. Sea condition will be very rough to high along and off these coasts. Fishermen are advised not to venture into the sea along and off these coasts.

FORMATION OF A SIGNIFICANT TROPICAL CYCLONE IS POSSIBLE WITHIN
135 NM EITHER SIDE OF A LINE FROM 7.2N 85.1E TO 9.6N 82.8E WITHIN
THE NEXT 12 TO 24 HOURS. AVAILABLE DATA DOES NOT JUSTIFY IS-
SUANCE OF NUMBERED TROPICAL CYCLONE WARNINGS AT THIS TIME. WINDS
IN THE AREA ARE ESTIMATED TO BE 25 TO 30 KNOTS. METSAT IMAGERY AT
110600Z INDICATES THAT A CIRCULATION CENTER IS LOCATED NEAR 7.8N
84.7E. THE SYSTEM IS MOVING NORTHWARD AT 05 KNOTS.
2. REMARKS: THE AREA OF CONVECTION PREVIOUSLY LOCATED NEAR 6.5N
84.1E, IS NOW LOCATED NEAR 9.4N 83.8E, APPROXIMATELY 285 NM EAST-
NORTHEAST OF COLOMBO, SRI LANKA. RECENT ANIMATED MULTISPECTRAL
SATELLITE IMAGERY INDICATES CONTINUED CONSOLIDATION OF DEEP
CONVECTION ABOUT AN AREA OF STRENGTHENED LOW LEVEL TURNING. THERE IS
SIGNIFICANT UNCERTAINTY IN THE LOCATION OF THE LOW LEVEL CIRCULATION
CENTER (LLCC) GIVEN THAT IT IS NOT YET FULLY CONSOLIDATED AND IT IS
OBSCURED BY HIGH CLOUDS. A 111530Z ASCAT IMAGE CONFIRMS A NORTH-
SOUTH ELONGATION OF THE LLCC WITH 20 TO 30 KNOT NORTHEASTERLY FLOW
POSITIONED TO SUPPORT FURTHER CONSOLIDATION OF THE LLCC. DEEP
CONVECTION CONTINUES TO BE FUELED BY STRONG RADIAL AND POLEWARD
OUTFLOW MECHANISMS. VERTICAL WIND SHEAR IS ALSO ASSESSED AS LOW.
MAXIMUM SUSTAINED SURFACE WINDS ARE ESTIMATED AT 25 TO 30
KNOTS. MINIMUM SEA LEVEL PRESSURE IS ESTIMATED TO BE NEAR 1001 MB.
THE POTENTIAL FOR THE DEVELOPMENT OF A SIGNIFICANT TROPICAL CYCLONE
WITHIN THE NEXT 24 HOURS IS GOOD.
Here's the predicted and traced path of "96B"...